Impacts of stratospheric polar vortex changes on tropospheric blockings over the Atlantic region

In recent years, extreme weather events associated with atmospheric blocking in the northern extratropics have become more frequent. This study has revealed the impacts of the stratospheric polar vortex on the blockings over the North Atlantic sector, using both reanalysis data and large-ensemble experiments performed by general circulation model. It is found that a weak stratospheric polar vortex (WPV) can cause more blockings to be generated over Greenland and move more westward than normal, while a strong stratospheric polar vortex (SPV) can cause more blockings to be generated over the south of Greenland and Western Europe. The stratospheric polar vortex could influence blocking anomalies by modulating both synoptic-scale eddy and planetary wave activities. Under WPV conditions, the generation of synoptic-scale eddies is suppressed due to decreased upper-troposphere background baroclinicity, which is favorable for positive geopotential height anomalies and more blockings over Greenland. Additionally, WPV can suppress the planetary wave train that is accompanied with lower pressure center over Greenland, further contributing to the positive geopotential height anomalies and more blockings over Greenland. The abovementioned processes under SPV conditions are nearly opposite to those under WPV conditions.